Small Engine Carburetor Troubleshooting
- Tank - Outlet hole inside the tank blocks with grit stopping or slowing fuel flowing to the carburettor
- Cap - Cap allows the tank to breath, when the cap vent fails it seals the tank stopping fuel flow
- Lines - Leak at connection points and on occasion can block stopping fuel flow
- Tap - Leak, causing fire risk
- Fuel filter - Block or slow fuel flow to the engine
- Pump - Fail causing a no start (Not fitted to all mowers)
- Carburettor - Block, under fuel and over fuel causing no start or poor running
- Fuel solenoid - Fail, stopping fuel flow (Not fitted to all mowers)
- Intake manifold - Leak, causing engine surging
- Refuelling can - Often the source of the dirt
- Air filter - Can block causing no start or poor running with black smoke
What Is Gumming?
How Does it Happen?
Gas turns to a gel and blocks everything up - ya nasty.
When it's bad, I prefer to replace the carb. Cleaning doesn't guarantee that you get it all, then your tearing it down again.
Don't even think about it, Order a new one!
Symptoms Of Carburetor Faults
Where to start?
Carburetor Fault Finding
Carburettors aren't expensive or difficult to fit. Sometimes it's better to just go ahead and replace the whole unit.
Carburettors do wear out and I replace lots of them.
Remove the connector to test for the click sound, or use a test light to check for power. Briggs and Kohler solenoids shown here.
Fuel Bowl Clean
The carburettor is located behind the air filter and you usually have enough room to work without removing any other components.
Turn the gas off, if you don't have a tap, use a grips to squeeze the line.
This type bowl doesn't have a solenoid.
Remove this bowl by removing the bowl bolt. An o ring gasket is used to seal the bowl to the carburettor.
Usually it stays on the carburettor side and that's OK, you can leave it there.
Clean the bowl, and when refitting use some lube on the o ring seal to prevent pinching.
This carburettor has a fuel solenoid.
To remove it, disconnect the wire connector and use an open ended wrench between the bowl and solenoid.
Sometimes you can just turn the bowl by hand.
Remember to lube the gasket when refitting the bowl.
Often no matter how careful you are, the bowl gasket will leak gas, if so, the only fix is to replace it.
Remove the fuel bowl drain bolt, which on some models its also the fuel solenoid.
Your bowl may have a bolt or two screws, and in some cases the whole bowl will come off.
Allow the fuel in the carburettor to drain out, catch in a suitable container and have some old rags handy.
If you have any doubts about fuel quality, drain the tank and fill with fresh gas.
Carburettor bowl type with two screws can be tricky to remove, so if that's your type just remove the solenoid, allow the gas to drain, reassemble and test. Often this is enough to fix the problem.
But before you reassemble check fuel flow on which ever type bowl you have.
Turn the fuel on: If fuel flows - Refit fuel bowl bolt and test mower.
If no fuel flow - Move on to Fuel flow check.
Testing Fuel Flow
Gravity Fuel System
Pump Fuel System
A gas tank needs to breath, when fuel leaves the tank it needs to be replaced with air.
A sealed tank will prevent fuel from flowing. Make sure you have gas in the tank.
Remove gas cap and check flow.
Check the fuel tank for grit - the out let hole is small and blocks easily. You may have to remove tank to clean thoroughly.
Examine the fuel lines from the tank to the carburettor, checking for kinks or damage.
Some fuel filters will be a see through bottle type, if its dirty - Change it. Arrow to carb.
If you can, remove the bowl - when the float is in the dropped position gas should flow.
Remove the float and needle, check condition. A worn needle turns pink in colour.
The needle seals the flow of gas when the float is in the up position.
A worn needle can block flow or cause gas to leak into the oil.
When this happens, I prefer to replace the complete carburettor.
Blow some carb cleaner into the needle seat on the carburettor.
Still no flow - Remove & clean carburetor, consider replacing complete unit.
Some carburetors have the seal in the tip of the needle and others have the seal in the carburetor. Carb removed for demo.
The pumped system is as said very similar. Check that the gas filter supply to the pump is OK.
The fuel pump, operates by the pulsing of crankcase pressure which is supplied by the hose pipe seen in the centre of the lower picture.
Check this pipe is secure and undamaged, sometimes they perish.
To test the pump - Remove the output line on the left and crank over the engine. No fuel flow - Replace pump.
If you've got poor running or carb gumming, it's time to treat her to a shinny new carburettor.New Carburettor
Remove & Clean Carburettor
Remove air filter and engine plastic cover.
Remove choke cable.
Turn off gas and remove fuel line. If you don't have a gas tap use a grips to gently squeeze the line.
Remove intake pipe.
Unplug solenoid valve and remove both carburettor bolts.
Take note of linkage, spring and gasket locations and orientation.
Unplug spring and lever.
Remove the float by sliding the pin out and removing the needle.
When worn the needle seal turns pink. Carburettor kits will include new bowl gaskets and needle seal.
Remove the main jet with a flat screwdriver.
Jets are made from brass which is a soft metal and will damage easily. Be sure the screwdriver is a good fit.
The dirt collects in the jet, it has small port holes which fuel flows through.
Clean the jet really well, the port holes may not look dirty, but a build up around them makes them smaller and restricts gas flow.
Use a strand of wire from a wire brush and run it through the holes.
The bowl gasket may be distorted or perished. Over-tightening or pinching will cause it to leak.
To avoid damage, lube o ring on reassembly.
When removing the fuel/air mix screw, count how many turns it takes to remove, and refit to the same number.
Use a good quality carb cleaner and compressed air if available.
Spray all passages and port holes.
A new carburettor makes a bit difference, cleaning won't guarantee it runs sweet.
So, if cleaning doesn't work out, go ahead and treat you mower to a new carb.
When rebuilding, replace gas filter. Clean your gas can and fill with fresh gas.
If your storing the mower for periods longer than a month, use a gas stabiliser. It will prevent gumming.
- No staring
- Lack power
- Gas level too low
- Gas tap off
- Blocked gas tank
- Blocked gas filter
- Blocked float needle
- Blocked carburettor jet
- Blocked gas lines